1. Field of the Invention
The present invention generally relates to reducing spin-down time in disk drives, and more particularly to an apparatus and method for reducing the time a spindle motor takes to spin-down in disk drives by pulse braking during a spin-down operation performed by the disk drive.
2. Description of the Prior Art
In hard disk drives, data is stored on magnetic media disks in concentric data tracks, which are divided into groups of data sectors. Disks are typically stacked on a spindle assembly. The spindle assembly is mechanically coupled to a spindle motor which rotates the disks at a high rate of speed. A spindle motor driver typically includes power field effect transistors (FETs) to drive the spindle motor. A microprocessor is typically employed to ascertain when to apply a run signal, a coast signal, or a brake signal to the spindle motor driver to control the operation of the spindle motor.
The torque developed (T.sub.d) by the spindle motor is given by the following Equations I:
Equations I EQU T.sub.d =Kt*I.sub.m EQU Kt=Ke(Kt in Nm/A; Ke in V/ rad/sec) EQU Bemf=Ke*.omega. EQU V.sub.applied =V.sub.source -Bemf=I.sub.m *(R.sub.m +R.sub.fet) EQU I.sub.m =(V.sub.source -Ke*.omega.)/(R.sub.m +R.sub.fet)
Where:
Td is the torque developed by spindle motor; PA1 Kt is the Torque constant of spindle motor; PA1 I.sub.m is spindle motor current; PA1 Ke is the Voltage constant of the spindle motor i.e. the Bemf factor; PA1 Bemf is the back emf of the spindle motor; PA1 .omega. is the rotational velocity of the spindle motor; PA1 V.sub.applied is the voltage applied to the spindle motor; PA1 V.sub.source is the voltage of the spindle motor source supplying power to the spindle motor; PA1 R.sub.m is the total resistance of the spindle motor and wire connections between the spindle motor and the power FETs ; and PA1 R.sub.fet is the resistance in the power FETS that are turned on for controlling current I.sub.m flowing through the spindle motor. PA1 .omega..sub.0 is the rotational velocity of the spindle motor at the start of dynamic braking; PA1 J is moment of inertia of the spindle assembly.
During a dynamic braking of the spindle motor during a spin-down operation of the disk drive, windings of the spindle motor are shorted. With the windings shorted, there is no V.sub.source therefore, a negative I.sub.m and reverse braking T.sub.d are developed as given by the following Equations II:
Equations II EQU I.sub.m (During Dynamic Braking)=-Bemf/(R.sub.m +R.sub.fet)=(-Ke*.omega.)/(R.sub.m +R.sub.fet) EQU T.sub.d (During Dynamic Braking)=Kt*I.sub.m =Kt*(-Ke*.omega.)/(R.sub.m +R.sub.fet)=(-Kt.sup.2 *.omega.)/(R.sub.m +R.sub.fet)
During dynamic braking, the rotational speed of the spindle motor at a given time (.omega.(t)) is given by the following Equations III:
Equations III EQU Td=.omega.J=(-Kt.sup.2 *.omega.)/(R.sub.m +R.sub.fet) EQU .omega.=(-Kt.sup.2 *.omega.)/(J*(R.sub.m +R.sub.fet)) EQU .omega.+[(Kt.sup.2 *.omega.)/(J*(R.sub.m +R.sub.fet))]=0
Solving in the frequency domain: EQU S*.omega.(s)-.omega.(0)+kt.sup.2 *.omega.(s)/(J*(R.sub.m +R.sub.fet))=0 EQU .omega.(s)[S+kt.sup.2 /(J*(R.sub.m +R.sub.fet))]=.omega.(0) EQU .omega.(s)=.omega.(0)/[S+Kt.sup.2 /(J*(R.sub.m +R.sub.fet))]
Using the Laplace Transform to solve for the time domain: EQU .omega.(t)=.omega..sub.0 e.sup.-kt.spsp..sup./j*(R.sbsp.m.sup.+R.sbsp.fet.sup.)
Where:
Power dissipation in the windings of the spindle motor are given by the following Equation IV:
Equation IV EQU P=I.sub.m.sup.2 *R.sub.m
Power dissipation in the power FETs are given by the following Equation V:
Equation V EQU P=I.sub.m.sup.2 *R.sub.fet
As can be shown by Equations I-V above, if dynamic braking is used during the initial stages of a spin-down operation in the disk drive, the power dissipation, which is proportional to the square of I.sub.m, is very high due to the high value of .omega.. This very high power dissipation would cause a corresponding heating of the spindle motor windings, which can result in heat related breakdown of the spindle motor components, including the spindle motor windings. Moreover, the power FETs employed to drive the spindle motor are typically especially susceptible to heat related breakdown from heat generated by the power dissipated in the power FETs.
Present disk drives prevent heat related breakdown of components in the spindle motor and the spindle motor driver power FETs by coasting to a safe rotational velocity .omega. or safe Bemf level before dynamically braking to .omega.=zero. One conventional way of coasting to a safe .omega. is for the microprocessor, which provides the coast and brake signals to the spindle motor driver, to provide the coast signal for a fixed delay interval after the start of a spin-down operation before providing the brake signal. Alternatively, the microprocessor reads the magnitude of .omega. from a speed detector/monitor and provides the coast signal until .omega. reaches the safe value. The coast period can be quite long. For example in a disk drive having a spindle motor that operates at 7200 revolutions per minute (RPMs), the coasting time can be six seconds or greater depending on inertial mass of the spindle system and drag torque. It is known to increase the RPM of the spindle motor to reduce rotational latency and increase disk transfer rate in the disk drive. However, as disk drives employ spindle motors operating at higher RPMs, a longer time period is needed to coast until a safe value of .omega. is reached. This long coast period not only results in a significantly longer spin-down operation, it also can cause transducer head and disk surface wear, which can reduce the mean time between failures (MTBF) of the disk drive.
When power supply energy fails, certain disk drives employ an emergency spin-down operation for generating an emergency auxiliary power supply during the spin-down of the spindle motor in response to the power failure. U.S. Pat. No. 5,504,402 to Menegoli (the '402 patent) and U.S. Pat. No. 5,495,372 to Bahlmann et al. (the '372 patent) describe such emergency auxiliary power supplies, which are energized by the spindle motor. However, the '402 patent and '372 patent are not directed to a normal spin-down operation where the microprocessor typically controls the spin-down of the spindle motor.
For example, in the '402 patent, a disk drive employs the kinetic energy stored in the rotating spindle assembly mass during a power failure to generate power to charge a voltage supply capacitor which provides power to a voice coil motor (VCM) to park the read-write head. A hardware circuit controls a switch, which switches on to short the windings of the spindle motor in order to provide a current path for current in the windings produced from the Bemf of the spindle motor. A current comparator compares the current in the windings to a reference current and turns off the switch to float the windings when the current rises above a maximum current threshold. A voltage comparator compares the voltage across the voltage supply capacitor with a reference voltage threshold and the hardware keeps the switch in the off position to continue floating the windings until the load discharges the voltage supply capacitor to a level below a comparator voltage threshold.
The period for shorting the windings depends on the maximum current threshold that is fixed in the hardware circuit. Also, the period for floating the windings depends on the voltage threshold that is fixed in the hardware circuit. Because the floating period depends on the voltage threshold for the voltage supply capacitor, the floating period can result in a longer spin-down operation. A longer spin-down time can be used for exploiting the kinetic energy stored in the spindle motor assembly mass to safely spin-down the disk drive. However, a longer spin-down operation can also cause transducer head and disk surface wear.
For reasons stated above, there is a need to reduce the time required to stop the spindle motor during the spin-down operation performed by the disk drive. In particular, there is a need to reduce the spin-down time without causing heat related breakdown of components in the spindle motor and the spindle motor driver power FETs. There is also a need to reduce the spin-down time without increasing the cost of the disk drive.